There have now been many studies describing how the RNA of SARS-CoV-2 can be detected in wastewaters (sewage) and even in some environmental samples. Most of these have been the subject of some discussion on this blog.
However, to understand public health implications, it is also necessary to understand whether any of that RNA is associated with intact viable viruses (virions) in these environments. While there are important insights available from studies with other enveloped viruses, very few observations are available from SARS-CoV-2 directly.
This recent (not yet peer reviewed) preprint paper takes an important step down that road. The paper is titled “Presence and vitality of SARS-CoV-2 virus in wastewaters and rivers”.
In this study, the authors evaluated the presence of SARS-CoV-2 RNA in raw and treated sewage from Milan, Italy. But in addition to that, they also tested for “virus vitality” before and after sewage treatment in relation to the “intrinsic persistence of SARS-CoV-2”.
In order to evaluate the vitality of SARS-CoV-2, a viral isolation protocol was conducted using a culture of Vero E6 cells, a monkey kidney cell line. Following culturing, vitality was assessed daily by screening cells for cytopathic effects.
The vitality of SARS-CoV-2 in sewage, either raw or treated samples, was determined to be “not significant”, despite the likely high numbers of RNA copies present in the samples. No cytopathic effects on Vero E6 cells were detected at 48 h and 72 h after inoculation.
In this study, the time delay from faeces emission (a toilet flush) to the arrival at the sewage treatment plant was estimated to be about 6-8 hours. Climatic conditions were mild (15-20°C) in the weeks of sampling activities. Based on previous observations regarding coronavirus survival, the authors note that these conditions “probably did not favour the survival of SARS-CoV-2” from the point of emission up to the point of sample collection, at the sewage treatment plants.
Since no virus vitality was observed, either before or after treatment, no conclusions can be drawn regarding the inactivation of viruses during the sewage treatment processes themselves.
But the authors do conclude that potential infection risks due to accidental contacts with sewage “seems to be negligible”, at least for the systems that they investigated. They state that from an environmental point of view, sewage treatment plants “should not constitute a significant source of infective SARS-CoV-2”.
This paper is a single (currently non-peer reviewed) study. There are many potential uncertainties, including the suitability of the wastewater extraction, culturing and cytopathic effect tests for SARS-CoV-2. Nonetheless, it’s a useful contribution to a growing understanding about the significance of this virus in municipal sewage.
Rimoldi, S. G., Stefani, F., Gigantiello, A., Polesello, S., Comandatore, F., Mileto, D., Maresca, M., Longobardi, C., Mancon, A., Romeri, F., Pagani, C., Moja, L., Gismondo, M. R. and Salerno, F. (2020) Presence and vitality of SARS-CoV-2 virus in wastewaters and rivers. medRxiv, 2020.05.01.20086009.